There is an increasing interest in monitoring body functions for various applications. For example, there is an interest in monitoring heart rate and breathing characteristics for relaxation, exercise and medical applications.
It has been proposed to measure body sounds by placing a microphone in the ear canal. Indeed body sounds, are transmitted through the body via bone conduction, among others. Interesting sounds that can be captured inside the ear canal include heart sounds, breathing sounds and motion sounds such as footsteps. It has been found that it is possible to record such body sounds inside the ear canal by making use of what is known as the occlusion effect. The occlusion effect refers to the phenomenon that inside an occluded ear canal, bone conducted sounds are perceived stronger than in an open ear canal. Besides this perceptual aspect, this effect can also be measured in terms of an increased sound pressure for low frequencies inside the ear canal. The explanation for this phenomenon is given in the article “Bone Conduction” by J. Tonndorf; in J. Tobias (ed.), Foundations of modern auditory theory, New York: Academic press, p. 197-237. For sounds inside the ear canal, the open end of the canal results in a high pass characteristic. As soon as the ear canal is sealed, the high pass characteristic is lost and the sound pressure level of low frequency sounds in the ear canal increases. More details of the occlusion effect with respect to different types of seals are provided in the article “A model of the occlusion effect with bone-conducted stimulation” by S. Stenfelt and S. Reinfeldt; International Journal of Audiology, vol. 46, p. 595-608, 2007. In this article, the occlusion effect is measured from 100 Hz upward, and it is found to extend up to 2 kHz, for some cases.
Research has been undertaken in positioning a microphone in the ear canal and sealing the ear to provide the occlusion effect. FIG. 1 illustrates an example of an ear microphone system where the microphones and seal are integrated in an earpiece such that the positioning of the earpiece in the ear both positions the microphone in the ear canal and seals the ear canal.
However, in order to be able to sufficiently clearly record bone-conducted body sounds in the ear canal using a microphone, the canal needs to be sealed properly. This seal will result in the occlusion effect which introduces a significant increase of the measured sound pressure level of the bone-conducted body sounds with respect to an open ear canal. As microphones suffer from self noise and therefore have a limited dynamic range, a significant sound pressure level at the microphone position is required to capture the desired body sounds. Thus, when using an ear microphone for recording body sounds, such as heart sounds and breathing sounds, the occlusion of the ear canal needs to be sufficiently good enough to provide a high enough sound-pressure level for body sounds in the ear canal. This requires an effective seal of the ear canal with respect to the external world to be present. If the sealing is not sufficient, the body sounds reduce dramatically in level and it becomes difficult or even impossible to derive specific information from the body sounds. Furthermore, the seal not only provides an increase in the level of the body sounds but also an attenuation of external sounds thereby improving signal to noise ratios.
Therefore the quality of the seal of the ear canal is very important for applications measuring body sounds in the ear canal. However, for examples such as e.g. that of FIG. 1, this requires that the earpieces are positioned correctly in the ear to provide a tight seal. As it may be performed by an inexperienced user, the positioning may often be suboptimal.
Accordingly an approach for estimating the quality of the seal of an ear canal would be advantageous. In particular an approach allowing increased flexibility, facilitated implementation, improved accuracy, and/or improved performance would be advantageous.